The quasi-public agency that administers the program said a file containing the names, addresses and Social Security numbers of customers and businesses was posted on the MTC’s Web site for 50 minutes June 25 before being taken down. The data breach affected most of the people who applied for the program. As of May 31, 846 residents had submitted applications.

A security consultant hired by the agency found that one user accessed the file during the incident.

“I sincerely apologize to our customers for this incident and any inconvenience or concern it may cause,” Massachusetts Technology Collaborative deputy executive director Philip Holahan said in a statement. “We are taking all possible steps to protect our customers’ information and ensure that such a breach does not happen again.”

The agency will provide free credit-monitoring services to those affected by the breach and will review its internal data security and encryption protocols. It has also informed the state attorney general’s office as well as the Office of Consumer Affairs and Business Regulation, per an executive order signed by Gov. Deval Patrick last year.

Launched in 2008, the Commonwealth Solar program provides rebates to individuals, businesses and communities that install solar photovoltaic systems on their property. It is the major financial driver behind a state goal to install 250 megawatts of solar in Massachusetts by 2017.